Electron beam microscope and method for operating it

Abstract

An electron beam microscope comprises an electron beam source 3, a magnetic objective lens 23 with poles 35, 37, a scintillator 61 with bore diameter D, a first electrode 80, a second electrode 97, and a potential supply device 11 for these elements. The bore diameter D of the scintillator is smaller than the bore diameter of the second electrode 97. Furthermore, |E(z) / r(z)| ≤ L for all points z on an axis of symmetry 29 from a region [z1, z2] near the scintillator. Here, E(z) is the field strength of the electric field at point z, r(z) is the radius of curvature of an equipotential line 91 of the electric field at point z, and L is equal to 3.0 kV / mm. 2 .

Description

[0001] The present invention relates to electron beam microscopes and methods for operating electron beam microscopes.

[0002] The invention relates in particular to electron beam microscopes comprising an electron beam source, a magnetic objective lens, a specimen holder, and an electron detector. The electron beam source generates an electron beam whose electrons are accelerated and which is focused by the magnetic objective lens into a plane of the specimen. The specimen holder holds the specimen to be examined in the plane of the specimen such that the focused electron beam strikes the surface of the specimen. The electron beam striking the specimen generates electrons that exit the specimen, such as secondary electrons and backscatter electrons. These electrons are detected by the electron detector. The detected electron intensity allows conclusions to be drawn about the structure of the specimen at the point where the electron beam strikes the specimen.The electron detector comprises a converter arrangement onto which the electrons generated at the object strike, producing signals that represent the intensity of the electrons produced at the object by the incident electron beam and can be evaluated by the control unit of the electron beam microscope. The converter arrangement can include a scintillator body onto which the electrons generated at the object strike, producing light. Such an electron detector then further comprises a light detector that detects the light produced by the scintillator arrangement and converts it into electrical signals that can be evaluated by the control unit of the electron beam microscope. Electron microscopes of this type are described, for example, in DE 10 2006 043 895 A1 and DE 10 2011 080 341 A1.

[0003] The converter arrangement can also include a semiconductor detector, onto whose semiconductor material the generated electrons strike and with which they generate the electrical signals that can be evaluated by the control of the electron beam microscope.

[0004] In some situations, it is desirable to detect a large proportion of the backscattered electrons generated by the object. These electrons have a higher kinetic energy than the secondary electrons that are also generated when they exit the object. To detect as many of these backscattered electrons as possible, it is advantageous to position the scintillator body of the scintillator array close to the object plane.

[0005] From US 9 029 766 B2, an electron beam microscope of this type is known, which has a scintillator arrangement with a scintillator body located close to the object.

[0006] It has become apparent that this conventionally known electron beam microscope does not meet certain expectations.

[0007] Accordingly, it is an object of the present invention to propose an electron beam microscope comprising an electron beam source, a magnetic objective lens, and a converter arrangement located near an object plane, and exhibiting improved properties. Furthermore, it is an object of the present invention to propose a method for operating such an electron beam microscope.

[0008] According to the invention, an electron beam microscope is proposed, comprising an electron beam source for generating an electron beam, a magnetic objective lens for generating a magnetic field focusing the electron beam, electrodes for generating an electric field to slow down the electrons of the electron beam, and a converter arrangement of an electron detector.

[0009] The magnetic objective lens comprises a magnetic coil and a yoke with a first pole and a second pole extending around an axis of symmetry. The electron beam can be focused by the objective lens onto the surface of an object held in a specimen holder of the electron microscope. The electric field, which decelerates the electrons of the electron beam, accelerates electrons generated at the object by the electrons striking the object, allowing them to strike the converter assembly of the electron detector with increased kinetic energy.

[0010] According to exemplary embodiments, the converter arrangement has a circular bore centered on the axis of symmetry, the smallest diameter of which lies in a converter plane orthogonal to the axis of symmetry. The converter arrangement comprises at least one converter that generates light or electrical signals from incident electrons.

[0011] The electron beam microscope further comprises a first electrode, which has a circular bore centered on the axis of symmetry on one side facing the converter arrangement and is located closer to the electron beam source than the converter arrangement, and a second electrode, which has a circular bore centered on the axis of symmetry on one side facing the converter arrangement and is located further away from the electron beam source than the converter arrangement.

[0012] The electron beam microscope further comprises a potential supply device configured to apply pairwise different electrical potentials to the first electrode, the second electrode, and the converter assembly in order to generate the electric field that decelerates the electrons of the electron beam. In addition to the first and second electrodes, the converter assembly thus provides a further electrode for shaping the electric field that decelerates the electrons of the electron beam. The geometry of the first electrode, the second electrode, and the converter assembly, as well as the electrical potentials applied to the first electrode, the second electrode, and the converter assembly by the potential supply device, determine the properties of the electric field generated by these elements, which is penetrated by the electrons of the electron beam on their way to the object.Furthermore, the portion of the electrons generated at the object that hits the converter arrangement passes through part of this field.

[0013] In exemplary embodiments, the smallest diameter of the converter assembly's bore is smaller than the diameter of the second electrode's bore. The diameter of the converter assembly's bore can be chosen to be small in order to provide the largest possible surface area of ​​the converter assembly upon which the electrons generated at the object can strike and be detected. Conversely, the diameter of the second electrode's bore is larger than the smallest diameter of the converter assembly's bore to prevent an excessive number of electrons generated at the object from striking the second electrode instead of the converter assembly.

[0014] According to exemplary embodiments, the diameters of the bores of the first electrode, the second electrode, and the third electrode, the distances between the first electrode and the converter arrangement and between the converter arrangement and the second electrode, as well as the electrical potentials applied to these elements, are selected such that, in a region [z1, z2], the relation | E(z) / r(z) | < L holds for all z. Here, z represents the points from the region [z1, z2] on the axis of symmetry. z1 is a point on the axis of symmetry that defines a boundary of the region [z1, z2] and is located at a distance from the converter plane equal to 0.35 times the smallest diameter of the bore of the converter arrangement in the direction towards the first electrode.z2 represents a point on the axis of symmetry that defines the other boundary of the region [z1, z2] and is located 0.35 times the smallest diameter of the converter assembly bore in the direction of the second electrode from the converter plane. E(z) represents the field strength of the electric field at point z. r(z) represents the radius of curvature of an equipotential line of the electric field passing through point z in a plane containing the axis of symmetry. Finally, L represents a limit value equal to 3.0 kV / mm. 2 In the above-mentioned relation, the magnitude of E(z) / r(z) is used to ensure that the comparison with the limit is independent of the signs of E(z) and r(z).

[0015] The region [z1, z2] is a range of points on the axis of symmetry between the boundaries z1 and z2. For example, if a coordinate system with its origin is placed at the intersection of the axis of symmetry and the surface of the object, such that the z-axis of the coordinate system coincides with the axis of symmetry, then the z-coordinate of point z2 is smaller than the z-coordinate of point z1. The converter plane, i.e., the plane orthogonal to the z-axis where the bore of the converter assembly has its smallest diameter, lies exactly midway between z2 and z1.

[0016] The equipotential surfaces of the generated electric field are essentially rotationally symmetric about the axis of symmetry, since the bores and other axially adjacent geometry of the elements that essentially determine the electric field—namely, the converter assembly and the first and second electrodes—are also essentially symmetric about the axis of symmetry. Thus, the curvature of the equipotential surfaces of the electric field on the axis of symmetry can be described by the curvature of the equipotential lines of the electric field in any plane containing the axis of symmetry. The curvature of the equipotential line at a specific point can, in turn, be described by its radius of curvature, which is defined as the radius of the circle that approximates the equipotential line at that point. It should be noted here that the bores of the first and second electrodes and the converter assembly are advantageously circular.However, deviations from a circular shape are possible, as long as the other performance requirements of the electron beam microscope are met. These deviations may be due to manufacturing limitations or other reasons. For example, the shape of the bores can be a polygon, such as a dodecagon.

[0017] The requirement that the relation | E(z) / r(z) | ≤ L holds in the region [z1, z2] means that the curvature of the equipotential surfaces on the axis of symmetry is small in the region of the bore and near the bore of the converter assembly. The bore of the converter assembly is penetrated by the electron beam focused on the object and, as mentioned above, advantageously has a comparatively small diameter. If the equipotential lines of the electric field were strongly curved in the region around the bore, this electric field would influence the focusing of the electron beam directed at the object.Furthermore, the electron beam directed at the object will not pass exactly through the bore of the converter assembly at the axis of symmetry in all operating modes of the electron beam microscope when the electron beam is scanned across the object to acquire an electron microscopic image, nor when using rocking-beam methods. In these cases, the electric field with its strongly curved equipotential lines would further negatively affect the focusing of the electron beam directed at the object. The curvature of the equipotential surfaces of the electric field, which delays the electrons of the electron beam, could also be reduced by increasing the smallest diameter of the bore of the converter assembly, while maintaining the same voltages applied to the first electrode, the converter assembly, and the second electrode.However, electrons generated at the object whose trajectories cross the converter plane at a small distance from the axis of symmetry can then only be detected to a lesser extent by the converter arrangement. Thus, adhering to the aforementioned relationship allows the diameter of the converter arrangement's bore to be kept small and ensures good focusing of the particle beam at the object even in strong electric fields that slow down the electrons of the electron beam.

[0018] According to exemplary embodiments, the electric field strength on the axis of symmetry in the converter plane is greater than or equal to 0.5 kV / mm or greater than or equal to 1.0 kV / mm. An advantageous electric field strength on the axis of symmetry in the converter plane lies in a range between 0.5 kV / mm and 5 kV / mm, and particularly in a range between 1.0 kV / mm and 2.5 kV / mm.

[0019] According to exemplary embodiments, the limit value L is equal to 1.0 kV / mm. 2 and in particular equal to 0.5 kV / mm 2 which leads to even smaller curvatures of the equipotential surfaces of the electric field in the area of ​​the bore of the converter arrangement.

[0020] According to exemplary embodiments, the converter arrangement, viewed in a cross-section containing the axis of symmetry, has a wedge-shaped form whose extension in a direction parallel to the axis of symmetry increases with increasing distance from the axis of symmetry. In other words, the converter arrangement in this cross-section has a shape that tapers towards the axis of symmetry. Since the entire surface of the converter arrangement is at the same electrical potential, the equipotential surfaces of the electric field run parallel to the surface in its immediate vicinity. By shaping the converter body, it is therefore possible to influence the orientation of the equipotential surfaces of the field even at locations far from the surface of the converter arrangement.In particular, the aforementioned wedge-shaped design of the converter body in cross-section makes it possible to keep the curvature of the equipotential surfaces in the area around the converter plane low in order to achieve the advantages mentioned above.

[0021] The wedge-shaped cross-section of the converter body can be described by a wedge angle α, which can be measured in a ring-shaped region of the converter assembly. This ring-shaped region has an inner diameter on both surfaces of the converter assembly that is up to 3 mm larger than the smallest diameter of the converter assembly's bore. The ring-shaped region often does not begin at the smallest diameter of the converter assembly's bore, as the converter assembly may have an intentional or manufacturing-related rounding at this point. The outer diameter of the ring-shaped region can be 3 mm larger than the inner diameter. In this ring-shaped region, the two surfaces of the converter body, viewed in cross-section, extend in a straight line to such an extent that an angle can be observed between the directions of extension of the surfaces.The two surfaces of the converter body can extend at an angle to each other that lies in a range between 8° and 75°, or in a range between 10° and 65°, or in a range between 15° and 40°.

[0022] Furthermore, the aforementioned wedge-shaped cross-section of the converter arrangement need not be arranged symmetrically to the converter plane. According to exemplary embodiments, the first surface of the converter arrangement, which is associated with the first electrode, extends at a first angle to the converter plane, and the other surface of the converter arrangement extends in the ring region at a second angle to the converter plane, the second angle being at least 10° greater than the first angle. In particular, the first surface can extend parallel to the converter plane in the ring region.

[0023] According to exemplary embodiments, H ≤ 0.5 * Ds, in particular H ≤ 0.2 * Ds, where Ds is a maximum extent or outer diameter of the converter arrangement in a direction orthogonal to the axis of symmetry, and H is a maximum extent of the converter arrangement in a direction parallel to the axis of symmetry. This means that the converter arrangement has a comparatively small extent in the direction of the axis of symmetry and is thin in this direction.

[0024] According to exemplary embodiments, the shape of the converter arrangement is particularly thin in the area near the bore. This can be described by the requirement that the converter arrangement has an extension h at the previously explained inner edge of the ring area in a direction parallel to the axis of symmetry, for which the following holds: h ≤ 0.7 * D, in particular h ≤ 0.5 * D and in particular h ≤ 0.25 * D, where D is the smallest diameter of the bore of the converter arrangement.

[0025] According to exemplary embodiments, the converter arrangement comprises at least one scintillator body formed from a scintillator material that generates light upon contact with incident electrons. The electron beam microscope further comprises a light detector configured to detect the light generated by the scintillator body and convert it into electrical signals.

[0026] According to other exemplary embodiments, the converter arrangement comprises a semiconductor detector with a semiconductor material in which the incident electrons generate electrical signals that are output by the semiconductor detector.

[0027] According to exemplary embodiments, the converter arrangement is held at the first or the second pole end of the objective lens.

[0028] Embodiments of the invention are explained in more detail below with reference to figures. Fig. Figure 1 shows a schematic sectional view of an electron beam microscope according to a first embodiment. Fig. 2 shows a partial view of the in Fig. 1 Electron beam microscope details of an objective lens and an electron detector. Fig. Figure 3 shows a schematic sectional view of a partial view of an electron beam microscope according to a second embodiment.

[0029] Fig. Figure 1 shows a schematic sectional view of an electron beam microscope according to a first embodiment. Fig. Figure 2 is a sectional view of part of the electron beam microscope, showing magnified details of an objective lens and an electron detector.

[0030] The electron beam microscope 1 comprises an electron beam source 3 with an electron emitter 5 and an extractor electrode 7, between which an electrical voltage is applied to extract electrons from the electron emitter 5, forming an electron beam 9. An electrical potential U5 is applied to the electron emitter 5, which is provided by a potential supply system 11. The potential supply system 11 is part of a control unit 12 of the electron beam microscope 1. The electron beam 9 emerges from a Fig. The electron beam enters the beam tube 15 at its upper end 13, passes through it, and exits at its lower end 17 before striking an object 19 to be examined. The beam tube 15 has an electrically conductive inner lining that is at an electrical potential U1 provided by the potential supply system 11. The potential difference U1 - U5 is, for example, 8 kV or more, so that the electrons of the electron beam 9 pass rapidly through the beam tube 15 between the first end 13 and the second end 17 with increased kinetic energy.

[0031] Object 19 is held on an object holder 21, to which an electrical potential U4 is supplied by the potential supply system 11, so that object 19 is also at the potential U4. The potential difference U4 - U5 defines the kinetic energy with which the electrons of the electron beam 9 strike object 19. Here, the difference U4 - U5 is smaller than the potential difference U1 - U5, so that the electrons of the electron beam 9 are decelerated between the second end 17 of the beam tube 15 and the surface of object 19, as will be explained below.

[0032] The electron beam microscope 1 further comprises a magnetic objective lens 23 to focus the electron beam 9 in an object plane 25. The object holder 21 positions the object 19 such that its surface essentially coincides with the object plane 25, so that the electron beam 9 is focused at an object location 27 on the surface of the object 19.

[0033] The magnetic objective lens 23 has an axis of symmetry 29 and two magnetic coils 31 and 32 surrounding the axis of symmetry 29 of the objective lens 23. Excitation currents are supplied to these coils by the control unit 12 to generate a magnetic field that focuses the electron beam. The magnetic coils 31 and 32 are surrounded by a yoke 33, which has a cross-section of Fig. 1 has a first pole end 35, a second pole end 37, and a third pole end 36. The yoke 33 and the pole ends 35, 37, and 36 surround the axis of symmetry 29 in a ring-like fashion. The pole ends 35, 37, and 36 are arranged at a distance from one another, such that the current flowing in the magnet coil 31 generates a magnetic field symmetrical with respect to the axis of symmetry 29 in the vicinity of pole ends 35 and 37, and the current flowing in the magnet coil 32 generates a magnetic field symmetrical with respect to the axis of symmetry 29 in the vicinity of pole ends 37 and 36. These two magnetic fields superimpose in a region near the axis of symmetry 29 to form a magnetic field that focuses the electron beam 9.The electron beam microscope 1 includes beam deflectors 38 and 39, which are controlled by the control unit 12 to deflect the electron beam 9 and thus move the object location 27, where the electron beam 9 hits the object 19, for example line by line over an area of ​​the object 19 and thus scan this area of ​​the object 19 with the electron beam 9.

[0034] The electrons of the electron beam 9 striking object 19 at object location 27 generate electrons through interaction with object 19. These electrons are emitted from the object and are detected by a first electron detector 41 and a second electron detector 43. During the scanning of object 19 with the electron beam 9, the detection signals S1 generated by electron detector 41 and S2 generated by electron detector 43 are recorded by the control system to ultimately generate an electron beam microscopy image of the scanned area of ​​object 19.

[0035] Trajectory 45 represents, by way of example, an electron emerging from object 19 at object location 27, which is detected by the first electron detector 41, and trajectory 47 represents, by way of example, an electron emerging from object 19 at object location 27, which is detected by the second electron detector 43.

[0036] The second electron detector 43 comprises a scintillator body 49, which is formed from a scintillator material that generates light upon contact with electrons. The scintillator body 49 is attached to an optical fiber 51 that guides the light generated by the scintillator body 49—exiting the scintillator body 49 and entering the optical fiber 51—to a light detector 53. Lines 55 represent such light. The light detector 53 generates electrical signals S2 from the detected light, which can be evaluated by the control unit 12 of the electron beam microscope 1. These electrical signals represent the intensity of the electrons generated by the electron beam 9 at the object location 27, which exit the object 19 in such directions and with such kinetic energy that they can strike the scintillator body 49.The scintillator body 49 and the light guide 51 have a bore 57 to allow the passage of the electron beam 9.

[0037] The first electron detector 41 comprises a converter assembly 60 and a light detector 63. The converter assembly 60 includes a scintillator body 61, onto which electrons 45 strike and generate light 65. This light propagates within the scintillator body 61, exits the scintillator body 61 into the vacuum at a light exit surface 67, and strikes a light entry surface 69 of the light detector 63 to be detected. The light detector 63 generates the electrical signals S1, which are transmitted from the objective lens 23 to the control unit 12 of the electron beam microscope 1 via lines 70 located within the objective lens 23. The control unit can evaluate these electrical signals.These electrical signals S1 represent the intensity of electrons generated by the electron beam 9 at the object location 27 and exiting the object 19 with such energy and in such directions that they can hit the scintillator body 61.

[0038] The converter assembly 60 is mounted on the first pole 35 of the yoke of the magnetic objective lens. The light detector 63 is mounted on the second pole 37 of the yoke 33 of the magnetic objective lens 23. The light-entry surface 69 of the light detector points away from the object plane 25 and towards the electron beam source 3.

[0039] The first electron detector 41 further comprises a reflective surface 72, which is provided at the second pole end 37 of the magnetic objective lens 23. The reflective surface 72 surrounds the electron beam 9 in a ring-like shape and symmetrically about the axis of symmetry 29. At the reflective surface 72, a portion of the light emerging from the light-exit surface 67 of the scintillator body 61, which does not directly strike the light detector 63, is reflected such that the light reflected at the reflective surface 72 strikes the light-intake surface 69 of the light detector 63 to be detected by it. An exemplary light beam 73 is shown in Fig. Figure 2 shows that the mirror 71 increases the detection probability of electrons 45 striking the scintillator body 61 by the light detector 63.

[0040] The beam tube 15 has a diameter D1 of 4.6 mm at its lower second end 17, which is close to the converter arrangement 60, and is arranged centrally with respect to the axis of symmetry 29. The lower end of the beam tube 15 thus forms a first ring electrode 80 opposite the converter arrangement 60, with a bore of diameter D1, which is at the potential U1.

[0041] The beam tube 15 is surrounded by electrical insulation 81, which in turn is surrounded by an electrically conductive sleeve 83. The scintillator body 61 of the converter assembly 60 is attached to the lower end 85 of the sleeve. An electrical potential U2 is supplied to the sleeve by the potential supply device. The surfaces of the scintillator body 61 are provided with an electrically conductive layer that electrically contacts the sleeve 83, so that the converter assembly 60 is also at the electrical potential U2. The scintillator body 61 has the shape of an annulus, which has a polygonal cross-section containing the axis of symmetry 29. The smallest diameter D of a bore in the annulus is 1.2 mm. A plane orthogonal to the axis of symmetry 29, which contains in the cross-section the location where the bore has the smallest diameter D, is called a converter plane 87.

[0042] A surface 89 of the scintillator body 61, which is assigned to the first ring electrode 80, extends parallel to the converter plane 87 and is located at a distance d1 of 0.4 mm from the first electrode 80. A surface 92 of the scintillator body 61, which points away from surface 89 and is close to the axis of symmetry 29, extends at an angle α of 25° to surface 89. In the representation of the Fig. 2. The extent of the scintillator body increases continuously in the direction parallel to the axis of symmetry 29 with increasing distance from the axis of symmetry 29. The shape of the scintillator body is wedge-shaped in its region near the axis of symmetry 29. The angle α can be measured in a suitable ring region in which the two surfaces 89 and 92 each have a sufficiently straight cross-section to allow the angle to be determined. This ring region has an inner diameter Di and an outer diameter Do. The inner diameter Di can be up to 3 mm larger than the smallest diameter D of the bore of the converter assembly 60, and the outer diameter Do can be 3 mm larger than the inner diameter Di.The shape of the scintillator body in its region near the axis of symmetry 29 is not only wedge-shaped but also pointed and thin, in that the extent of the scintillator body 61 in the direction parallel to the axis of symmetry 29 is small near the bore of the scintillator arrangement 60. For example, the scintillator body has an extent of 0.1 mm in the direction parallel to the axis of symmetry 29 at the inner edge with diameter Di.

[0043] Another surface 93 of the scintillator body 61, pointing away from surface 89 of the scintillator body 61, extends parallel to surface 89.

[0044] The electrically conductive coating of the light-emitting surface 67 of the scintillator body 61 is transparent, while the other electrically conductive coatings of surfaces 89, 92 and 93 of the scintillator body 61 are light-reflecting. Surfaces 92 and 93 are the electron-receiving surfaces of the converter arrangement 60, i.e., the surfaces on the converter arrangement 60 onto which the electrons generated by the electron beam 9 strike in order to be detected by the scintillator body 61 generating light with these electrons.

[0045] The extent H of the scintillator body 61 and the converter arrangement 60 in the direction of the axis of symmetry 29 is 1 mm, and the extent Ds of the scintillator body 61 and the scintillator arrangement 60 in the direction of the converter plane 87 is 9.0 mm.

[0046] The light detector 63 is embedded in a recess 77 provided in the second pole end 37. The light detector 63 is at an electrical potential U3, which is equal to the electrical potential of the first pole end 35 and the yoke 33 and is provided by the potential supply system 11. An inner edge 79 of the second pole end 37, which is aligned with the axis of symmetry 29, extends symmetrically around the axis of symmetry 29. The second pole end 37, which is at the electrical potential U3, thus forms a second ring electrode 97, the bore of which is defined by the inner edge 79. The diameter D2 of the bore of the second ring electrode 97 is 5 mm. The distance d2 in the direction of the axis of symmetry 29 of the ring electrode 97 from the converter assembly 60 is 1.2 mm.

[0047] In the section view in Fig. Figure 2 shows equipotential lines 91 of an electric field generated by the first ring electrode 80, the converter arrangement 60, and the second ring electrode 97 in a central area around the axis of symmetry 29, for the case where U1 is 8.0 kV, U2 is 7.0 kV, and U3 is 0 V. The electric potential U4 of the object is also 0 V. Here, the equipotential lines 91 represent the equipotential surfaces of the electric field in the sectional view.

[0048] From the Fig. Figure 2 shows that the curvature of the potential lines 91 in the region of the bore of the converter arrangement 60 is relatively small. To quantify this, the following procedure can be used: On the axis of symmetry 29, a region [z1, z2] is defined between two points z1 and z2 on the axis of symmetry 29. Point z1 lies between the converter plane 87 and the electron beam source 3 and is located 0.35 times the smallest diameter D of the bore of the converter arrangement 60 from the converter plane 87 in the direction of the first ring electrode 80. Point z2 lies between the converter plane 87 and the object 19 and is also located 0.35 times the smallest diameter D of the bore of the converter arrangement 60 from the converter plane 87 in the direction of the second ring electrode 97.Then, in this region [z1, z2], the maximum of | E(z) / r(z) | is calculated, where E(z) is the field strength of the electric field at point z, and r(z) is a radius of curvature of an equipotential line of the electric field at point z in a plane containing the axis of symmetry, which is the plane of the drawing. Fig. 2. If the maximum determined in this way is smaller than a limit value L, the curvature of the equipotential lines 91 in the area of ​​the bore of the converter arrangement 60 is relatively small.

[0049] A limit value L of 3.0 kV / mm can be used. 2 or 1.0 kV / mm 2 or 0.5 kV / mm 2 be elected.

[0050] In Fig. 2. This maximum occurs for the equipotential line with an electric potential of approximately 6.5 kV, and the radius of curvature of this equipotential line at the axis of symmetry 29 is approximately 3.4 mm. The radius r of the circle that approximates this equipotential line where the equipotential line intersects the axis of symmetry is therefore approximately 3.4 mm. The electric field strength E(z) at this point is approximately 1.3 kV / mm. Thus, in Fig. 2 as a maximum for | E(z) / r(z) | in the region [z1, z2] a value of approximately 0.4 kV / mm 2 .

[0051] For the calculation of the properties of the electric field on the symmetry axis 29 and the determination of the quantities E(z) and r(z) a simulation program can be used, to which the geometry of the first ring electrode 80, the converter arrangement 60 and the second ring electrode 97 as well as the electric potentials applied to these elements are provided.

[0052] Furthermore, it is possible to use such a simulation program to vary the geometry of these elements and the electrical potentials applied to these elements until a particularly small value is reached for the expression | E(z) / r(z) | in the entire range [z1, z2].

[0053] A suitable simulation program for these calculations is available under the name "COMSOL" from Comsol Multiphysics GmbH, 37073 Göttingen, Germany.

[0054] Further embodiments of the electron beam microscope are shown below with reference to the figures. Components are shown that, with regard to their structure or function, are components of the electron beam microscope shown in the figures. Fig. The embodiments described in point 1 correspond to those described above, with the same reference numerals, but distinguished by an additional letter. For an understanding of the structure and function of these components, please refer to the entire preceding description.

[0055] Fig. Figure 3 shows a partial cross-sectional view of an electron beam microscope according to a second embodiment.

[0056] A in Fig. The electron beam microscope 1a shown in Figure 3 has a similar structure to the one shown in Figure 3. Fig. 1 and Fig. 2. Electron beam microscope 1. The electron beam microscope 1a differs from the electron beam microscope 1 of the Fig. 1 and Fig. 2 solely by the design of a converter arrangement 60a for detecting electrons generated at an object. Accordingly, the electron beam microscope 1a comprises an electron beam source, a beam tube 15a, an objective lens 23a, and a specimen holder, as previously described. The objective lens 23a, in turn, has a yoke with a first pole end and a second pole end 37a. At a lower end 17a of the beam tube 15a, a first ring electrode 80a with a diameter D1 of 5 mm is formed. The second pole end 37a, arranged symmetrically with respect to an axis of symmetry 29a, has an inner rim 79a with a diameter D2 of 6 mm, thus forming a second ring electrode 97a.

[0057] A converter arrangement 60a is arranged along the axis of symmetry 29a between the first ring electrode 80a and the second ring electrode 97a. The converter arrangement 60a has a bore centered with respect to the axis of symmetry 29a, which has a minimum diameter D of 1.2 mm. The converter arrangement 60a includes a semiconductor detector 62, which is arranged in a converter plane and onto which the electrons 45a generated at the object impinge and generate electrical signals in the semiconductor material of the semiconductor detector 62. These signals are transmitted from the semiconductor detector 62 via lines that are Fig. 3, which are not shown, are output to a control unit of the electron beam microscope 1a. Not shown is in Fig. Figure 3 is also a support for the converter arrangement 60a between the first ring electrode 80a and the second ring electrode 97a. However, the semiconductor detector 62 can be supported, for example, at the second pole end 37a by suitable elements made of insulating material. In this case, a support for the semiconductor detector 62 can have rounded elements, as is the case, for example, in Fig. 2 at the lower end 85 of the sleeve 83 for holding the scintillator body 60 is shown in order to avoid flashovers in a strong electric field,

[0058] A distance d1 in the direction of the axis of symmetry 29a between the first ring electrode 80a and the semiconductor detector is 0.6 mm. A distance d2 between the semiconductor detector 62 and the second ring electrode 97a is 0.8 mm. As in the first embodiment, an electrical potential U1 of 8 kV is applied to the first ring electrode 80a. The second ring electrode 97a is also applied an electrical potential U3 of 0 V, as in the first embodiment. The electrical potential applied to the object is also 0 V. An electrical potential U2 of 4.5 kV is applied to the converter arrangement 60a. This electrical potential can be supplied to the converter arrangement 60a via the leads of the semiconductor detector 62. The semiconductor detector 62, arranged in the converter plane 87a, is particularly thin and has a thickness H of 0.3 mm in the direction of the axis of symmetry 29a.The extent Ds of the semiconductor detector 62 in a direction orthogonal to the axis of symmetry 29a is 8 mm.

[0059] Also in Fig. Figure 3 shows equipotential lines 91a of an electric field generated by the first ring electrode 80a, the converter arrangement 60a, the second ring electrode 97a, and the object, as well as the electric potentials U1, U2, U3, and U4 applied to these elements, in a region near the axis of symmetry 29a. It is evident that the equipotential line for the electric field strength of 4.5 kV exhibits almost no curvature near the converter plane 87a. The condition | E(z) / r(z) | ≤ 0.5 kV / mm 2 is thus in the Fig. The situation described in point 3 is certainly fulfilled.

[0060] In summary, an electron beam microscope disclosed herein comprises an electron beam source 3, a magnetic objective lens 23 with poles 35, 37, a scintillator 61 with bore diameter D, a first electrode 80, a second electrode 97, and a potential supply device 11 for these elements. The bore diameter D of the scintillator is smaller than the bore diameter of the second electrode 97. Furthermore, |E(z) / r(z)| ≤ L for all points z on an axis of symmetry 29 from a region [z1, z2] near the scintillator. Here, E(z) is the field strength of the electric field at point z, r(z) is a radius of curvature of an equipotential line 91 of the electric field at point z, and L is equal to 3.0 kV / mm. 2 .

Claims

[1] Electron beam microscope, comprising: an electron beam source (3) configured to produce an electron beam (9); a magnetic objective lens (23) for generating a magnetic field focusing the electron beam (9), wherein the magnetic objective lens (23) has a magnetic coil (31) and a yoke (33) with a first pole end (35) and a second pole end (37), each extending around an axis of symmetry (29); a converter arrangement (60) having a circular bore centered on the axis of symmetry (29), having its smallest diameter (D) in a converter plane (87) orthogonal to the axis of symmetry (29) and comprising at least one converter which generates light (65) or electrical signals with incident electrons (45); a first electrode (80) which has a circular bore centered on the axis of symmetry (29) on one side assigning to the converter arrangement (60) and is arranged closer to the electron beam source (3) than the converter arrangement (60); a second electrode (97) which has a circular bore centered on the axis of symmetry (29) on a side assigning to the converter arrangement (60) and is arranged further away from the electron beam source (3) than the converter arrangement (60); and a potential supply device (11) configured to supply pairwise different electrical potentials (U1, U2, U3) to the first electrode (80), the second electrode (97) and the converter arrangement (60) in order to generate an electric field that slows down electrons of the electron beam (9); wherein the smallest diameter (D) of the bore of the converter arrangement (60) is smaller than a diameter (D2) of the bore of the second electrode (97); where | E(z) / r(z) | ≤ L for all z from a range [z1, z2], where z is a point on the axis of symmetry (29); z1 is a point on the axis of symmetry (29) which is a distance from the converter plane (87) of 0.35 times the smallest diameter (D) of the bore of the converter arrangement (60) in the direction towards the first electrode (80); z2 is a point on the axis of symmetry (29) that is 0.35 times the smallest diameter (D) of the bore of the converter arrangement (60) in the direction of the second electrode (97) from the converter plane (87); E(z) is the field strength of the electric field at point z; r(z) a radius of curvature of an equipotential line (91) of the electric field at point z; and L is a limit value equal to 3.0 kV / mm 2 is. [2] Electron beam microscope according to claim 1, where L equals 1.0 kV / mm 2 and in particular equal to 0.5 kV / mm 2 is. [3] Electron beam microscope, comprising: an electron beam source (3) configured to produce an electron beam (9); a magnetic objective lens (23) for generating a magnetic field focusing the electron beam (9), wherein the magnetic objective lens (23) has a magnetic coil (31) and a yoke (33) with a first pole end (35) and a second pole end (37), each extending around an axis of symmetry (29); a converter arrangement (60) having a circular bore centered on the axis of symmetry (29), having its smallest diameter (D) in a converter plane (87) orthogonal to the axis of symmetry (29) and comprising at least one converter which generates light (65) or electrical signals with incident electrons (45); a first electrode (80) which has a circular bore centered on the axis of symmetry (29) on one side assigning to the converter arrangement (60) and is arranged closer to the electron beam source (3) than the converter arrangement (60); a second electrode (97) which has a circular bore centered on the axis of symmetry (29) on a side assigning to the converter arrangement (60) and is arranged further away from the electron beam source (3) than the converter arrangement (60); and a potential supply device (11) configured to supply pairwise different electrical potentials (U1, U2, U3) to the first electrode (80), the second electrode (97) and the converter arrangement (60) in order to generate an electric field that slows down electrons of the electron beam (9); wherein the smallest diameter (D) of the bore of the converter arrangement (60) is smaller than a diameter (D2) of the bore of the second electrode (97); wherein the converter arrangement (60), seen in a cross-section containing the axis of symmetry (29), has a wedge-shaped form, the extent of which in a direction parallel to the axis of symmetry (29) increases with increasing distance from the axis of symmetry (29). [4] Electron beam microscope according to claim 3, wherein the converter arrangement (60) has in the cross-section a first surface (89) assigning to the first electrode (80) and a second surface (92) assigning to the second electrode (97); wherein the first and second surfaces (89, 92) each have a ring region, the ring region having an inner diameter (Di) that is up to 3 mm larger than the smallest diameter (D) of the bore of the converter arrangement (60), and an outer diameter (Do) that is 3 mm larger than the inner diameter (Di); wherein the first surface (89) and the second surface (92) extend in the ring region, as seen in the cross-section, at an angle to each other of more than 8° and less than 75°, in particular of more than 10° and less than 65° and in particular of more than 15° and less than 40°. [5] Electron beam microscope according to claim 4, wherein the first surface (89) extends in the ring area, as seen in the cross-section, at a first angle to a plane orthogonal to the axis of symmetry (29); wherein the second surface (92) extends in the ring region, as seen in the cross-section, at a second angle to the plane orthogonal to the axis of symmetry (29); and where the second angle is at least 10° larger than the first angle. [6] Electron beam microscope according to claim 5, wherein the first surface (89) extends in the ring area parallel to the plane orthogonal to the axis of symmetry (29). [7] Electron beam microscope according to any one of claims 1 to 6, where H ≤ 0.5 * Ds, in particular H ≤ 0.2 * Ds, holds true Ds is a maximum extension of the converter arrangement (60) in a direction orthogonal to the axis of symmetry (29); and H is a maximum extent of the converter arrangement (60) in a direction parallel to the axis of symmetry (29). [8] Electron beam microscope according to any one of claims 1 to 7, wherein at a location (Di) of the converter arrangement (60) which is at a distance from the axis of symmetry (29) that is 0.1 mm or 0.3 mm greater than half the smallest diameter (D) of the bore of the converter arrangement (60), h ≤ 0.7 * D, in particular h ≤ 0.5 * D and in particular h ≤ 0.25 * D, holds, where D is the smallest diameter of the bore of the converter arrangement (60); and h is an extension of the converter arrangement (60) in a direction parallel to the axis of symmetry (29). [9] Electron beam microscope according to any one of claims 1 to 8, wherein the converter arrangement (60) comprises at least one scintillator body (61) which is formed from a scintillator material which generates light (65) when electrons (45) are incident on it. [10] Electron beam microscope according to any one of claims 1 to 9, wherein the converter arrangement (60) is held at the first or the second pole end (35, 37). [11] Electron beam microscope according to any one of claims 1 to 10, further comprising a light detector (63) configured to detect light (65) generated by the converter arrangement (60) and to convert it into electrical signals (S1). [12] Electron beam microscope according to claim 11, wherein the light detector is held at the first or the second pole end (35, 37). [13] Electron beam microscope according to one of claims 1 to 8, wherein the converter arrangement (60a) comprises at least one semiconductor detector (62). [14] Electron beam microscope according to any one of claims 1 to 13, wherein at least one of the following relations is satisfied: (U2−U3)≤(U1−U3) (U2−U3)≤0.3*(U1−U3); (U2−U3)≥2 kV; and (U2−U3)≤15 kV; where U1 is an electrical potential supplied to the first electrode (80), U2 is an electrical potential supplied to the second electrode (97), and U3 is an electrical potential supplied to the converter arrangement (60). [15] Electron beam microscope according to claim 14, where at least one of the following relations is satisfied: (U2−U3)≤0.9*(U1−U3) (U2−U3)≤0.5*(U1−U3); (U2−U3)≤3 kV; (U2−U3)≤9 kV. [16] Electron beam microscope according to claim 14 or 15, furthermore comprising an object holder (21) for holding an object (19) to be examined; wherein the potential supply device (11) is configured to supply an electrical potential to the object holder (21); where: |U4−U3|≥2 kV; where U4 is the electrical potential supplied to the object holder (21). [17] Electron beam microscope according to any one of claims 1 to 16, wherein the smallest diameter (D) of the bore of the converter arrangement (60) in the scintillator plane (87) is less than 2.4 mm and greater than 0.6 mm. [18] Electron beam microscope according to any one of claims 1 to 17, wherein the smallest diameter (D1) of the bore of the first electrode (80) is more than twice the diameter (D) of the bore of the converter assembly (60) in the converter plane (87). [19] Electron beam microscope according to any one of claims 1 to 18, wherein the diameter (D2) of the bore of the second electrode (97) is more than twice the diameter (D) of the bore of the converter assembly (60) in the converter plane (87). [20] Electron beam microscope according to any one of claims 1 to 18, where, measured in the direction of the axis of symmetry (29), a distance (d1) between the first electrode (80) and the converter arrangement (60) is smaller than a distance (d2) between the second electrode (97) and the converter arrangement (60). [21] Electron beam microscope according to claim 20, wherein the distance (d1) between the first electrode (80) and the converter arrangement (60) is more than 0.3 mm smaller than the distance (d2) between the second electrode (97) and the converter arrangement (60). [22] Electron beam microscope according to any one of claims 1 to 21, where at a location arranged on the axis of symmetry (29) and in the converter plane (87) one of the following relations is satisfied: 0.5 kV / mm≤E; 0.5 kV / mm≤E≤5 kV / mm; and 1.0 kV / mm≤E≤2.5 kV / mm. [23] Electron beam microscope according to any one of claims 1 to 22, wherein at a location on a surface of the converter arrangement (60) assigned to one of the second electrodes (97), which is at a distance from the axis of symmetry that is 1.0 mm greater than half the smallest diameter (D) of the bore of the converter arrangement (60), the following applies: E ≥ 0.5 kV / mm, in particular E ≥ 1.0 kV / mm, and in particular E ≥ 2.0 kV / mm. [24] Method for operating an electron beam microscope, in particular in combination with the electron beam microscope according to any one of claims 1 to 23, wherein the electron beam microscope comprises: an electron beam source; a magnetic objective lens comprising a magnetic coil and a yoke with a first pole end and a second pole end, each extending around an axis of symmetry; a converter arrangement comprising a circular bore centered on the axis of symmetry, having its smallest diameter in a converter plane orthogonal to the axis of symmetry and comprising at least one converter; a first electrode which has a circular bore centered on the axis of symmetry on one side facing the converter arrangement and is located closer to the electron beam source than the converter arrangement; a second electrode having a circular bore centered on the axis of symmetry on one side facing the converter arrangement and positioned further away from the electron beam source than the converter arrangement; and a potential supply device; wherein the smallest diameter of the bore of the converter assembly is smaller than the diameter of the bore of the second electrode; and the procedure includes: Generating a magnetic field with the objective lens; Supplying pairwise different electrical potentials to the first electrode, the second electrode and the converter arrangement to generate an electric field; Generating an electron beam with the electron beam source; Focusing the generated electron beam with the magnetic field; Slowing down the electrons of the electron beam with the electric field; Directing the slowed-down electrons of the electron beam towards an object; Generating signals with the converter using electrons that are generated on the object by the electrons of the electron beam directed at the object; Capturing the generated signals; where | E(z) / r(z) | ≤ L for all z from a region [z1, z2]; where z is a point on the axis of symmetry; z1 is a point on the axis of symmetry that is 0.35 times the smallest diameter of the bore of the converter arrangement in the direction of the first electrode from the converter plane; z2 is a point on the axis of symmetry that is 0.35 times the smallest diameter of the bore of the converter arrangement in the direction of the second electrode from the converter plane; E(z) is the field strength of the electric field at point z; r(z) is a radius of curvature of an equipotential line of the electric field through the point z in a plane containing the axis of symmetry; and L is a limit value equal to 3.0 kV / mm 2 is.

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